Big rigs with bombs are secretly cruising the interstate near you. But how safe are they from terrorists or accidents?

"Is that it?" My wife leans forward in the passenger seat of our sensible hatchback and points ahead to an 18-wheeler that's hauling ass toward us on a low-country stretch of South Carolina's Highway 125. We've been heading west from I-95 toward the Savannah River Site nuclear facility on the Georgia-South Carolina border, in search of nuke truckers. At first the mysterious big rig resembles a commercial gas tanker, but the cab is pristine-looking and there's a simple blue-on-white license plate: US GOVERNMENT. It blows by too quickly to determine whether it's part of the little-known US fleet tasked with transporting some of the most sensitive cargo in existence.

As you weave through interstate traffic, you're unlikely to notice another plain-looking Peterbilt tractor-trailer rolling along in the right-hand lane. The government plates and array of antennas jutting from the cab's roof would hardly register. You'd have no idea that inside the cab an armed federal agent operates a host of electronic countermeasures to keep outsiders from accessing his heavily armored cargo: a nuclear warhead with enough destructive power to level downtown San Francisco.

That's the way the Office of Secure Transportation (OST) wants it. At a cost of $250 million a year, nearly 600 couriers employed by this secretive agency within the US Department of Energy use some of the nation's busiest roads to move America's radioactive material wherever it needs to go—from a variety of labs, reactors and military bases, to the nation's Pantex bomb-assembly plant in Amarillo, Texas, to the Savannah River facility. Most of the shipments are bombs or weapon components; some are radioactive metals for research or fuel for Navy ships and submarines. The shipments are on the move about once a week.